Not Applicable.
The present invention relates to the modification of organic and inorganic fibers using plasma technology and microwave radiation. More specifically, the present invention relates to a method for producing an undulated surface on reinforcement fibers using microwave radiation and a non-uniform plasma energy.
Advanced structural composites are reinforced polymers constructed using a matrix material and one or more reinforcement elements, such as fibers, filaments, or elongated particles. They are generally lightweight and possess superior strength and elasticity over most metals, and are often used as structural members in the aerospace industry and in high-tech space applications. Advanced structural composites are also used in other broader commercial applications where low weight and high mechanical strength materials are required, such as tennis rackets, fishing poles and golf clubs.
In general, the mechanical properties of the composite depend primarily upon the reinforcement elements selected and their ability to interact with the matrix material, usually a polymeric resin. The intrinsic mechanical properties of these two constituents are very different and, therefore, each constituent serves a different function. The function of the matrix material is to bind the reinforcement elements together to form a coherent structure, and to provide a medium for transferring applied loads from one element to another. The matrix material also provides the composite with its high temperature mechanical properties, transverse strength and moisture resistance, and is a key factor in providing toughness, shear strength, and oxidation and radiation resistance. The matrix material also strongly influences the fabrication process and the associated parameters for forming intermediate and final products from the composite material.
The reinforcement constituent, on the other hand, functions as the composite""s load-bearing element. This is because the strength of the reinforcement material is generally many orders of magnitude greater than the matrix material. As a result, the matrix resin can generally tolerate higher levels of deformation than the reinforcement material. This higher tolerance allows the matrix system to distribute applied loads from one reinforcement element to another. For this reason, good bonds between the reinforcement elements and the matrix resin are extremely important for composites subjected to loads, particularly shear-critical loads.
If fibers are selected as the reinforcement element, a broad spectrum of fibers with variable mechanical properties can be used. For example, one commonly used fiber is the carbon fiber. Carbon fibers have a very high strength and/or stiffness when compared to polymeric resins. Other fibers include fibers made of glass, nylon, rayon, cellulose, aramide, polyethylene, polypropylene, silicon carbide and more.
Early studies with carbon and glass fiber have demonstrated that surface treatments can lead to improved interfacial adhesion and, thus, better mechanical composite properties. In the case of carbon fiber reinforcement, these surface treatments were targeted toward the improvement of the chemical bond between the carbon fiber and the epoxy matrix resin.
Fiber manufacturers have developed many fiber surface treatments to modify the characteristics of polymer surfaces and to enhance their adhesion to resin matrices. These technologies include anodic oxidation, electro-deposition, wet and dry oxidation, acid etching, low-energy plasma treatments, transcrystallinization, ion implantations, covalent bonding, etc. The basic principle of these technologies is to place chemically active groups on the surface of each fiber. These chemically active groups, in turn, react chemically with other groups in the surrounding matrix to form a strong mechanical bond and, thus, tie the fiber surface and the matrix together.
In low-energy plasma treatments, plasma generated photons and energy particles interact with the fiber surface, usually by free radical chemistry, to enhance the adhesive characteristics of the fiber. The use of low-energy plasma surface treatment is a well known technology, previously discussed at length by Werthelmer et al., xe2x80x9cPlasma Treatment of Polymers to Improve Adhesion,xe2x80x9d Adhesion Promotion Techniques: Technological Applications, 139-174 (Mittal and Pizzi, ed., 1999); J. C. M. Peng et al., xe2x80x9cSurface Treatment of Carbon Fibers,xe2x80x9d Carbon Fibers, Third Edition, 180-187 (J. B. Donnet et al., ed., 1998); L. H. Peebles, xe2x80x9cPlasma Treatment,xe2x80x9d Carbon Fibers Formation, Structure, and Properties, 128-135 (1995); Listen et al., xe2x80x9cPlasma Surface Modification of Polymers for Improved Adhesion: A Critical Review,xe2x80x9d J. Adhesion Sci. Technol., 7:10:1091-1127 (1993); and J. Delmonte, xe2x80x9cSurface Treatment of Carbon/Graphite Fibers,xe2x80x9d Technology of Carbon and Graphite Fiber Components, 189-191 (1981).
The use of a plasma surface treatment will generally result in a cleaning of the fiber""s surface; an ablation, or etching, of material from the fiber""s surface; a cross-linking or branching of the fiber""s near-surface molecules; and a modification of the fiber""s surface chemical structure. (See Werthelmer et al., supra at 145; and Listen et al., supra at 1096.) Each effect is always present to some degree, although to a variable extent depending upon the fiber substrate, the plasma gas chemistry, the plasma reactor design, and the overall operating parameters. Each of these effects also contributes in a synergistic manner to the enhancement of adhesion. For example, surface cleaning and ablation improves adhesion by removing organic contaminates and weak boundary layers from the fiber""s surface. Cross-linking improves adhesion by providing a thin cross-linked layer of molecules on the fiber""s surface which mechanically stabilizes the surface and serves as a barrier to inhibit low molecular weight molecules from diffusing into the fiber/matrix interface. Finally, chemical modification, the most dramatic and widely reported effect of plasma, improves adhesion by introducing to the fiber surface new chemical groups capable of interacting and covalently linking with the matrix resin to yield the strongest bonds.
It is also known that ablation may enhance the adhesive characteristics of some polymer surfaces by causing a change in the fiber""s surface morphology. This change is usually a result of the cleaning of badly contaminated surfaces, or the removal of weak boundary surface layers formed during the fabrication process, or the treatment of filled or semi-crystalline materials. In particular, plasma removes amorphous polymers many times faster than crystalline polymers or inorganic fillers. Therefore, the over-treatment of polymer surfaces containing zones of amorphous polymers may result in the ablated amorphous zones appearing as random valleys or pits. This change is believed to have the unexpected effect of improving the mechanical interlocking of the polymer surface, while increasing the polymer""s surface area available for chemical interactions.
Although it is known that some ablation of reinforcing fibers may improve composite properties, surface treatments for deliberately modifying the topography of fiber surfaces are very limited. This is because present methods generally only provide random ablative activity in those zones containing amorphous polymers, and often require over-treatment of the fiber in order to obtain the modification. Over-treatment, in turn, may also have the undesired effect of reducing the fiber""s diameter, resulting in a thin reinforcing fiber having significantly weakened bulk properties. In addition, the etching or pitting of the fiber may result in cornered edges, which may further reduce the bulk properties of the fiber, or create air traps which may interfere with effective fiber/resin binding.
The present invention is summarized in that a novel method is disclosed for producing an undulated surface on reinforcement fibers using a non-uniform microwave generated plasma and microwave radiation. In general, reinforcement fibers are introduced into an oxygen-free atmosphere under pressure, the fibers being under slight tension and at least partially stabilized, and then subjected to a microwave generated plasma flux and microwave radiation, the plasma flux and microwave radiation being varied over either space or time to produce the undulated surface. The plasma is varied by either modulating the power input of the microwave energy or the internal pressure of the plasma chamber in which the plasma is generated, or by altering the angle at which the microwave radiation reacts with the plasma chamber to generate the plasma. Preferably, the plasma is generated in a controlled oxygen free plasma chamber using an oxygen free gas capable of acting as a carrier for the effluents of the processing system. The microwave radiation is generated by a standard microwave generator capable of providing a power input of between 250 W and 100 kW, and more preferably between 750 W and 15 kW.
The undulated surface on the reinforcement fibers can be produced in a batch process, quasi-batch process, or a continuous process. In a continuous process, the intensity of the plasma and the microwave radiation is preferably varied over time to produce the undulated surface on the fiber as it is passed through the plasma field, the intensity variation being induced by either a pulsing of the microwave power level or a gradual change in the microwave power level or a modulation of the internal pressure of the plasma chamber. In a quasi-batch or a batch process the intensity of the plasma and the microwave radiation may be varied over either space or time to create the undulated surface, the variation over space being induced by passing the fiber over a region having a higher intensity of plasma energy or microwave radiation.
Another aspect of the present invention are the fibers produced by the method of the present invention, wherein the cross-sectional area of the fiber is reduced by up to about 50% of its original cross-sectional area, or preferably reduced by up to about 30% of its original cross-sectional area, or more preferably reduced by up to about 15% of its original cross-sectional area.
A principle object of the present invention is to provide a method for increasing the strength of advanced structural composites by improving the mechanical bonding between the composite""s resin matrix and its reinforcement fibers.
It is another object of the present invention to provide a novel method for producing an undulated surface on reinforcement fibers in order to improve the fibers"" ability to mechanical interlock with composite resins.
It is yet another object of the present invention to utilize plasma technology and electromagnetic radiation to enhance the mechanical binding properties of reinforcement fibers.
One advantage of the present invention is that the surface topography of the fiber can be modified using either a batch, quasi-batch, or continuous system.
Another advantage of the present invention is that the surface of the fiber can be undulated to desired parameters.
Yet another advantage of the present invention is that the use of both a plasma energy and a microwave radiation allows for efficient processing of the reinforcement fibers.
Other objects, features, and advantages will become apparent upon consideration of the following detailed description, drawings and examples.